The present invention relates to a method of producing hydrogen in which oxygen is separated from an oxygen containing feed by an oxygen transport membrane, the oxygen is reacted with a hydrocarbon and steam to produce a synthesis gas, and the hydrogen is separated from the synthesis gas through the use of a hydrogen transport membrane. More particularly, the present invention relates to such a method in which a hydrogen-depleted crude synthesis gas is combusted to heat the oxygen containing feed.
Hydrogen is currently used in the synthesis of many different industrial chemicals. It is expected that additional production of hydrogen will be required for fuel cells to be used in transportation and distributed power generation markets. Many of the current and future hydrogen requirements can be most economically met by the use of small-scale hydrogen plants having an output of less than about 4 million standard cubic liters per day. In this regard, the use of fuel cells in the distributed power generation market is projected to grow substantially over the next 10 to 20 years. It is expected that this market will require a large number of such small-scale hydrogen plants.
A well-known method for producing hydrogen is steam methane reforming. Hydrocarbons such as methane are reformed with steam in a steam methane reformer to produce a synthesis gas mixture containing hydrogen and carbon monoxide. In a shift reactor, carbon monoxide and steam are reacted to produce a hydrogen-rich gas containing hydrogen and carbon dioxide. The hydrogen-rich gas can be purified by pressure swing adsorption to recover pure hydrogen. As can be appreciated, the foregoing processes are conducted in large-scale installations that can be capable of producing more than 3 billion standard cubic liters of hydrogen per day.
Reactors have at least been proposed in the prior art in which steam, one or more hydrocarbons, and air are reacted to produce a synthesis gas. Hydrogen is separated from the synthesis gas by a hydrogen transport membrane. An example of such a reactor is disclosed in U.S. Pat. No. 5,741,474. In this patent, hydrogen is produced by reforming hydrocarbons with oxygen or air and steam to produce a crude synthesis gas containing hydrogen, carbon monoxide, water, and carbon dioxide. The hydrogen is recovered from the synthesis gas by use of a hydrogen transport membrane.
U.S. Pat. Nos. 4,810,485 and 5,637,259 also describe membrane reactors that integrate hydrogen generation with hydrogen separation by a membrane. In U.S. Pat. No. 4,810,485 a reactor is disclosed in which a hydrogen containing gas is produced by steam methane reforming or a water gas shift reaction and a hydrogen transport membrane is used to separate hydrogen from the hydrogen containing gas. U.S. Pat. No. 5,637,259 describes a tubular reactor and membrane to produce hydrogen from a synthesis gas produced within the reactor.
Hydrogen transport membranes, that are effective to separate hydrogen from hydrogen containing gases, include membranes made of metals or metal alloys, proton conducting ceramic materials and porous ceramic membranes. All of such membranes function at high temperatures.
In metal-based and porous ceramic membranes, hydrogen permeation is due to the higher hydrogen partial pressure on the retentate side as compared to the permeate side. Several examples of metal-based membranes in the prior art include U.S. Pat. Nos. 3,350,846, 5,215,729, and 5,738,708. The membranes of the foregoing patents are composite membranes in which a layer, formed of Group IVB or VB metals, is sandwiched between two layers of a metal selected from either palladium, platinum or their alloys. In U.S. Pat. No. 5,217,506, a composite membrane is disclosed that contains intermetallic diffusion barriers between two top layers and a central membrane layer to prevent diffusion of top metal layer into the central metal layer. The barrier is made from oxides or sulfides of molybdenum, silicon, tungsten and vanadium. U.S. Pat. No. 5,652,020 describes a hydrogen transport membrane comprised of a palladium layer deposited on porous ceramic support layer. U.S. Pat. No. 5,415,891 describes a porous ceramic membrane modified by either metallic oxide (e.g. aluminum or zirconium oxide) or non-metallic oxide (e.g. silicon oxide).
Proton conducting ceramic materials can be characterized as being either electrically-driven (a pure proton conductor) or pressure driven (a mixed conductor).
Electrically-driven membranes are pure proton conductors that do not have electrical conductivity. Such membranes need an external circuit to drive electrons from an anode surface of the membrane to cathode surface. One of the advantages of an electrically-driven membrane is that there is no need to maintain high pressure because electrical force can be used to transport hydrogen to the permeate zone and to produce pressurized hydrogen directly. A second advantage is the reduced need for a purge gas on the permeate side. Proton conducting ceramics suitable for high-temperature application include perovskite-type oxide based on cerates or zirconates as cited in H. Iwahara, xe2x80x9cHydrogen Pumps Using Proton Conducting Ceramics And Their Applicationsxe2x80x9d, Solid State Ionics 125 (1999), pp 271-278 (1999).
Pressure driven membranes capable of conducting both protons and electrons do not need external circuit and can operate in non-galvanic mode. Examples of mixed conducting, hydrogen transport membranes are disclosed in U.S. Pat. Nos. 6,066,307 and 6,037,514. U. Balachandran et al., xe2x80x9cDevelopment of Mixed-Conducting Ceramic Membrane for Hydrogen Separationxe2x80x9d, presented at the Sixteenth Annual International Pittsburgh Coal Conference Proceedings, Pittsburgh, Pa., Oct. 11-15, 1999 discloses that electronic conductivity can be increased by mixing metal powder with mixed conductors such as partially substituted perovskite-type oxides such as CaZrO3, SrCeO3 and BaCeO3.
Other prior art reactor designs, in addition to the hydrogen transport membrane, incorporate an oxygen transport membrane to produce oxygen for partial oxidation reactions that provide heat for the endothermic steam methane reforming reaction. For instance, in the reactor design shown in U.S. Pat. No. 6,066,307, hydrogen is produced from partial oxidation and steam methane reforming reactions of a hydrocarbon fuel, steam, and oxygen using a reactor containing oxygen transport membranes to produce the oxygen and hydrogen transport membranes to separate hydrogen from a crude synthesis gas. As the hydrogen is removed, the shift conversion reaction results in additional hydrogen generation. An oxygen containing feed, composed of air, is heated by three streams, composed respectively of oxygen-depleted air, hydrogen-depleted crude synthesis gas, and hydrogen returning from the reactor. In a reaction zone of the reactor, oxygen from the heated air permeates through the oxygen transport membrane and reacts with a mixture of a hydrocarbon containing fuel and steam to produce the synthesis gas. Hydrogen from the reaction zone permeates through the hydrogen transport membrane. The oxygen-depleted air, hydrogen-depleted crude synthesis gas, and the hydrogen are cooled to recover thermal energy and thereby heat the incoming feed and in turn help heat the membranes to their operational temperatures.
It is to be noted that oxygen transport membranes function by transporting oxygen ions, formed from oxygen at a surface of the membrane known as the cathode side, to the opposite surface of the membrane, known as the anode side. The oxygen molecule is reconstituted at the anode side and electrons lost from the oxygen ions upon reconstitution of the oxygen are transported to the cathode side for oxygen ionization.
There are membrane materials, referred to as mixed conductors, that can conduct oxygen ions as well as electrons. Various known perovskites are suitable for such purposes. There are also dual phase metal and metallic oxide combinations that can also be used. Examples of mixed conductors and dual phase combinations can be found in U.S. Pat. Nos. 5,702,999, 5,712,220 and 5,733,435. All of such membranes operate at an elevated temperature, between about 400xc2x0 C. and about 1000xc2x0 C. and in the pressure-driven mode, that is, the partial pressure of oxygen on the cathode side of the membrane is higher than on the anode side.
In all of the reactor designs discussed above heat produced from partial oxidation reactions of the hydrocarbon containing feed help to balance the endothermic heat requirements of reforming reactions and the heating requirements for hydrogen transport membranes and oxygen transport membranes where the same are employed. It is to be noted that hydrogen recovery is reduced to the extent that partial oxidation reactions are used to meet heating requirements due to the production of water and carbon dioxide in the crude synthesis gas resulting from such reactions. The reason for this is that the presence of water and carbon dioxide reduce the partial pressure of the hydrogen. In addition, increased carbon dioxide levels drives the shift conversion reaction in the reverse direction which reduces the hydrogen available for separation. Therefore, heat recovery from the discharged streams, for instance, hydrogen-depleted crude synthesis gas, hydrogen product streams, and oxygen depleted retentate streams, is particularly critical.
In the prior art, such heat recovery is through indirect heat exchange that is effectuated by the use of heat exchangers. Such heat exchangers add to the complexity and cost of the use of reactors discussed above and therefore, do not make them very amenable for use in small-scale hydrogen production. For instance, in the plant described in U.S. Pat. No. 6,066,307, three streams from the reactor, made up of an oxygen depleted retentate, hydrogen product, and hydrogen-depleted crude synthesis gas, must be separately cooled. Therefore, three additional, separate heat exchangers are required. Furthermore, two heat exchangers (economizer and boiler) are used for steam generation, one heat exchanger is required for preheating the hydrocarbon feed to the desulfurization temperature and one heat exchanger is needed to preheat the feed to reactor temperature. Thus, a total of seven heat exchangers are required to practically carry out the teachings of this patent.
Even in reactors that do not employ an oxygen transport membrane, there is still a practical requirement for the utilization of a number separate heat exchange devices. For instance, in U.S. Pat. No. 5,741,474, an annular combustion chamber surrounds the reactor to combust a mixture of part of the hydrocarbon feed to be reacted, the hydrogen-depleted crude synthesis gas, and part of the incoming air feed. In addition, two separate heat exchangers are employed to recover heat to separately heat the incoming air and hydrocarbon feeds to the reactor.
As will be discussed, the present invention provides a hydrogen generation method that inherently is less complex and therefore, more useful for small-scale hydrogen generation plants than prior art processes and methods discussed above.
The present invention provides a method of producing hydrogen in which oxygen is separated from an oxygen containing feed stream with an oxygen transport membrane to produce an oxygen permeate. A heated oxygen permeate is reacted with one or more hydrocarbons, contained in a hydrocarbon containing feed stream, and steam, contained in a steam feed stream, to produce a crude synthesis gas comprising hydrogen, carbon monoxide, water, and carbon dioxide. The hydrogen is separated from the crude synthesis gas in a hydrogen transport membrane to produce a hydrogen-depleted crude synthesis gas and a hydrogen permeate. A product stream is formed that contains hydrogen from the hydrogen permeate. A stream of the hydrogen-depleted crude synthesis gas in the presence of an oxygen-containing feed stream is combusted, thereby to form the heated oxygen-containing feed stream.
Separation of the oxygen from the oxygen containing feed stream forms an oxygen depleted retentate. The hydrocarbon containing feed stream can be preheated and the steam contained in the steam feed stream can be produced through indirect heat exchange with a retentate stream composed of the oxygen depleted retentate.
The partial pressure of hydrogen can be reduced through the use of a sweep gas composed of steam. The steam and water resulting from the use of the sweep gas can be removed from a hydrogen permeate stream composed of the hydrogen permeate to form the product stream. Preferably, the steam within the sweep gas stream is superheated, a make-up water stream, provided for make-up of the steam, is preheated, and the hydrocarbon containing gas stream is preheated through indirect heat transfer with the hydrogen permeate stream. The water is removed from the hydrogen permeate stream after the indirect heat transfer by condensing the water and separating the condensed water in a phase separator.
Ethane and other higher order hydrocarbons contained within the hydrocarbon containing feed stream can be prereformed to methane prior to reacting the oxygen permeate with the hydrocarbon and the steam. Other pretreatment can include removing sulfur from the hydrocarbon containing feed stream. Part of the hydrogen produced can be added to the hydrocarbon containing feed stream.
In a specific embodiment of the present invention, the oxygen can be separated from the oxygen containing gas and synthesis gas can be generated in a first reaction stage. A synthesis gas stream formed from the synthesis gas can be introduced into a second reaction stage to separate the hydrogen from the synthesis gas and thereby to produce the hydrogen-depleted crude synthesis gas. The steam for the steam feed stream can be produced through indirect heat exchange with the synthesis gas stream. In such embodiment, the hydrocarbon containing feed stream and an air stream to supply the air for combustion of the hydrogen-depleted crude synthesis gas are preheated through heat exchange with a retentate stream composed of an oxygen depleted retentate formed by separation of the oxygen from the oxygen containing feed stream. A make-up water stream, provided for make-up of the steam, is preheated, and the hydrocarbon containing gas stream is preheated through indirect heat transfer with the hydrogen permeate stream.
The oxygen containing feed stream can be compressed to a sufficiently high pressure to allow for the recovery of work. In this regard, separation of the oxygen from the heated oxygen containing feed stream produces an oxygen-depleted retentate. An oxygen-depleted retentate stream composed of the oxygen-depleted retentate is expanded with the performance of work. The work may be extracted as electrical power and/or drive a compressor or blower.
In any embodiment of the present invention, the hydrogen transport membrane can be a metal membrane or a proton conducting membrane or a porous ceramic membrane. The oxygen transport membrane can be formed from a mixed conductor membrane or a dual phase mixed metal and metal oxide membrane.
In absence of the present invention, the obvious choice for small-scale hydrogen production is to flare hydrogen-depleted crude synthesis gas after recovering the thermal energy. However, it is difficult to flare the gas because of its low thermal energy and low Btu content. In any event, flaring will represent an energy loss from the plant. In this regard, if the hydrogen-depleted crude synthesis gas is sent to the flare at a high temperature to facilitate combustion, there will be even greater losses of both thermal and fuel energy. Thus, the use in the present invention of hydrogen-depleted crude synthesis gas as a fuel to preheat air or other oxygen containing gas, not only utilizes both its fuel and thermal energy and solves problem of flaring of low grade fuel gas but also simplifies apparatus by reducing the number of heat exchangers. Furthermore, combustion of the hydrogen-depleted, crude synthesis gas produces more heat per mole of oxygen consumed than other fuels. Since the oxygen containing gas is the source of oxygen in the production of the synthesis gas, it is important to conserve oxygen molecules during combustion.